Charles T. Robbins

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A method has been developed for detecting tannin-binding proteins in the saliva of herbivores. The method is simple and requires only small quantities of crude saliva. The saliva of deer, a browsing ruminant, has been compared to that of domestic sheep and cow, which are grazing ruminants. The browser, which normally ingests dietary tannin, produces(More)
We tested the competing hypotheses that (1) nitrogen discrimination in mammals and birds increases with dietary nitrogen concentration or decreasing C:N ratios and, therefore, discrimination will increase with trophic level as carnivores ingest more protein than herbivores and omnivores or (2) nitrogen discrimination increases as dietary protein quality(More)
As a result of pioneering work of Hofmann (1973, 1989), nutritional ecologists classify ruminants into three feeding-type categories: browsers (“concentrate” feeders), grazers, and intermediate or mixed feeders. Hofmann proposed that these feeding types result from evolutionary adaptations in the anatomy of the digestive system and that one consequence is(More)
We quantified the amount, spatial distribution, and importance of salmon (Oncorhynchus spp.)-derived nitrogen (N) by brown bears (Ursus arctos) on the Kenai Peninsula, Alaska. We tested and confirmed the hypothesis that the stable isotope signature (δ15N) of N in foliage of white spruce (Picea glauca) was inversely proportional to the distance from(More)
Accurately predicting isotopic discrimination is central to estimating assimilated diets of wild animals when using stable isotopes. Current mixing models assume that the stable N isotope ratio (δ15N) discrimination (∆15N) for each food in a mixed diet is constant and independent of other foods being consumed. Thus, the discrimination value for the mixed(More)
Abstract Previous studies on wild black bears (Ursus americanus) have shown that skeletal muscle morphology, composition, and overall force-generating capacity do not differ drastically between seasons despite prolonged inactivity during hibernation. However, the amount and characteristics of the seasonal variations were not consistent in these studies. The(More)
Stable isotope signatures of lactating females and their nursing offspring were measured on 11 species, including herbivores, carnivores, hibernators, and non-hibernators. We hypothesized that: (1) nursing offspring would have stable isotope signatures that were a trophic level higher than their mothers, and (2) this pattern would be species-independent.(More)
We explored multiple linkages among grey wolves (Canis lupus), elk (Cervus elaphus), berry-producing shrubs and grizzly bears (Ursus arctos) in Yellowstone National Park. We hypothesized competition between elk and grizzly bears whereby, in the absence of wolves, increases in elk numbers would increase browsing on berry-producing shrubs and decrease fruit(More)
Disuse uncouples bone formation from resorption, leading to increased porosity, decreased bone geometrical properties, and decreased bone mineral content which compromises bone mechanical properties and increases fracture risk. However, black bear bone properties are not adversely affected by aging despite annual periods of disuse (i.e., hibernation), which(More)
North American porcupines (Erethizon dorsatum) subsist predominantly on low-protein, high-fiber, high-tannin diets. Therefore, we measured the porcupine's ability to digest dry matter, fiber, and protein by conducting digestion trials on eight natural forages and one pelleted ration varying in concentration of fiber, nitrogen, and tannins. On these diets,(More)